1. Field of the Invention
The invention relates to a mail-handling machine which processes a series of documents in order to produce a series of groups of documents, and particularly to an apparatus for determining whether a group of documents fed onto a processing track in pile form contains the correct number of documents.
2. Related Art
Mail-handling machines are well-known for automating accumulation of a group of inserts along an insertion track and insertion of the accumulated group of inserts into a series of envelopes. Such machines are taught, for example, in U.S. Pat. No. 4,955,185 to Haas, U.S. Pat. No. 4,697,246 to Zemke, and U.S. Pat. No. 4,223,882 to Stocker, which are incorporated herein by reference. In mail-handling machines of this type, it is known to provide a detector for sensing whether a document or group of documents which is fed onto an insertion track for insertion in an envelope contains more documents than expected (a "double document" situation) or less documents than expected (a "missed document" situation). Such situations might arise, for example, when there has been a misfeed at a feeder station along the group's insertion track, resulting in too few or too many documents being fed to a particular group. A detector for sensing such situations is commonly known as a "double/miss detector", and such detectors are generally positioned proximal to a feeder along the insertion track so as to perform measurements upon a document during its ejection from the feeder but before it is added to its respective group of documents along the insertion track. When a "double document" or "missed document" situation is detected, an error signal is sent to a machine control device which then performs a remedial operation, such as ejection of the error-causing document or documents or alerting of a machine operator so that the situation can be remedied manually.
In the device according to Zemke a Hall Effect type double/miss detector is employed to sense a double or miss situation in a document as the document is engaged by a feeder for feeding onto an insert track. The feeder comprises a gripper arm for grasping a single document and pulling it from the bottom of a stack of documents. The gripper arm further comprises a set of jaws between which the document-to-be-fed is grasped. A field-generating means is mounted on one jaw, while a Hall Effect sensor is mounted on the other jaw for sensing the flux density of the generated field, the flux density being a function of the relative displacement of the two jaws.
U.S. Pat. No. 4,471,954 to Bourg, which is incorporated herein by reference, teaches a document-handling machine employing a second type of double/miss detector. The machine according to Bourg utilizes an infrared type double/miss detector mounted in a passage through which a sheet being ejected from a feeder passes. An infrared light source is mounted on one wall of the passage, while an infrared detector is mounted on an opposite wall. As a sheet is ejected from the feeder, the measurement signal from the infrared detector is compared with a reference signal, and if the measurement signal differs from the reference signal beyond a predetermined deviation, an alarm signal indicating a double or miss situation is generated.
The document-handling machines of the prior art provide double/miss detection at a sheet-feeder output, and that detection is limited to sensing of whether a feeder is ejecting a single sheet, more than a single sheet, or no sheets at all. While the machines of the prior art perform their intended purpose admirably, they cannot detect a double/miss situation in a group of multiple documents wherein the expected number of documents in a particular group being detected may vary from that of other groups in a series of groups.